When a sheet-like member such as a check, a paper money, a printing paper or a manuscript is fed to recognize characters or the like provided on the sheet-shaped member, duplicate feeding in which a plurality of papers is simultaneously feed in an overlapped manner is required to be prevented.
In order to prevent the duplicate feeding, a technique utilizing ultrasonic wave has been known in which a ultrasonic transmitter and a ultrasonic receiver are disposed on opposite faces of a transfer passage for feeding a sheet-like member and, when ultrasonic wave emitted from the ultrasonic transmitter is received with the ultrasonic receiver, duplicate feeding is judged on the basis of an attenuation amount of the ultrasonic wave transmitting through the sheet-like member.
In the technique with the use of ultrasonic wave, in order to stably detect duplicate feeding, an amplification factor of the ultrasonic receiver is increased so as to detect a minute difference of an attenuation amount of the ultrasonic wave regardless of a condition of the sheet-like member such as difference of its thickness, a wrinkled state or a folded state.
However, when the amplification factor of the ultrasonic receiver is increased, an external noise is also amplified simultaneously and an S/N ratio is lowered. Therefore, a technique has been known in which, in order to efficiently enhance a reception level while restraining dispersion of element characteristics such as sensitivity, resonance frequency and directivity of the ultrasonic transmitter and the ultrasonic receiver, a frequency of the ultrasonic wave emitted from the ultrasonic transmitter is made variable and duplicate feeding is detected with the use of the optimum oscillation frequency.
Duplicate feeding detection which is disclosed in Japanese Patent Laid-Open No. 2006-298598 is performed in which an ultrasonic wave with an output voltage and an oscillation frequency having adjusted beforehand is emitted from an ultrasonic transmitter. The output voltage is adjusted so that a voltage is increased in a fixed frequency and the lowest voltage is set to be the output voltage when the reception level of the ultrasonic receiver becomes larger than a predetermined value. Further, the oscillation frequency is adjusted so that, in a case that the reception level of the ultrasonic receiver does not become more than the predetermined value even when the voltage of the ultrasonic transmitter is increased, the frequency is varied in a specified range and the step in which the voltage is increased is repeated again. The frequency when the reception level of the ultrasonic receiver becomes larger than the predetermined value is determined as the oscillation frequency. A combination of the lowest output voltage and the oscillation frequency whose reception level is high provides a good transmission efficiency for the ultrasonic wave and a high detection accuracy of duplicate feeding.
Duplicate feeding detection which is disclosed in Japanese Patent Laid-Open No. 2006-1691 utilizes that, when the frequency is continuously varied within a predetermined range, the sensitivity of the ultrasonic receiver becomes maximum at a certain frequency regardless of its element characteristics and, in this manner, the accuracy of duplicate feeding detection at the resonance point is enhanced.
However, in the duplicate feeding detection which is disclosed in the former Patent Reference, it is not considered that the resonance frequency varies due to temperature characteristic of the piezo-electric element. Therefore, even when the ultrasonic wave is emitted from the ultrasonic transmitter at a predetermined oscillation frequency regardless of a temperature condition, the output of the ultrasonic transmitter is decreased and thus the reception level of the ultrasonic receiver is also decreased to lower the S/N ratio. Further, the resonance frequency is shifted due to a temperature characteristic of the piezo-electric element and thus a portion with a high S/N ratio is not used in an actual using state.
Further, in the duplicate feeding detection which is disclosed in the latter Patent Reference, accuracy of duplicate feeding detection at a resonance point can be enhanced by sweeping of the frequency. However, duplicate feeding cannot be detected with a high degree of accuracy over a whole area of a sheet-like member to which duplicate feeding detection is to be performed and thus there is a possibility of erroneous detection according to a degree of overlapping of sheet-like members.